Free water and solids segregation have long been identified as the source of many problems in well bore cementations. Many SPE publications have discussed the role of free water development and solids segregation in "gas-cutting" of cement and inter-zonal communication. This is a particular problem in deviated well bores where the solids and free water have only to move a few inches to collect. The water can then slide along the upper formation face and the solids downward along the lower formation face. Free water can also collect and form "open pockets" in well bore irregularities.
The industry's awareness of this problem has resulted in concerted efforts to control free water development and sedimentation. While the API has recently been active in developing a more stringent "Operating Free Water Test", several individual companies within the industry have already developed their own review criteria. Most include some physical/visual inspection, some further specify minimum yield point YP or Ty values since certain rheologic properties control cement slurry stability. These YP or Ty values represent an extrapolated y intercept, typically using the shear stress/shear rate relationships present at higher shear rates, such as 1022 to 170 reciprocal seconds. However, the actual y intercepts and low shear rate fluid rheology are typically much lower since most cement slurries fail to follow "ideal" Bingham Plastic or Power Law behavior. Cement slurries typically have actual shear stress values falling well below projected values in the shear rate range of from 1022 to 170 reciprocal seconds. As a result, the API suggests use of shear rates of from about 511 to 170 reciprocal seconds to minimize non-model behavior at low shear rates.
Cement slurries are a suspension of solids in water. The solids are suspended by the chemical and physical interactions of these particles. Unfortunately, most of the cement modifiers used for fluid loss control and for ensuring adequate pumping time reduce low shear viscosity or disperse the cement particles. AMPS co-polymers (acrylomido methyl propane sulfonate copolymers) are particularly notorious for reducing low shear viscosity, yield intercept and progressive gels. Cement dispersants, i.e. sodium naphthalene sulfonate condensed with formaldehyde, and retarders i.e. lignosulfonates and organic acids, strongly disperse the cement particles. This dispersion reduces cement viscosity at all shear levels and greatly reduces yield point and progressive gelation. Low shear viscosity, yield point, and progressive gel strength development are key to proper solids support and free water control. A material that would boost these values would be most beneficial.
Past work at addressing this problem has focused on three approaches. One uses a heat-activated gellant, another uses materials that preferentially increase yield point, and the third uses progressive gel strength development. Heat-activated or latent solubility polymers do not yield during initial surface mixing, but increase overall viscosity as they yield at higher temperatures. This permits higher viscosity than would otherwise be possible and offsets to some degree thermal thinning. The problem with this approach is that the organic polymers have an upper temperature limitation of 250-300.degree. F. They also remain subject to thermal thinning. Therefore, to achieve adequate viscosity at 250.degree. F., the viscosity and friction pressure will be greater than desired at &lt;200.degree. F.
Materials such as Kelco's Biozan, bentonite and attapulgite have been used to preferentially boost cement slurry yield point and low shear viscosity. While they do increase plastic viscosity and apparent viscosity at the higher shear rates, greater gains are made in the yield point and progressive gels. Unlike the temperature activated materials, the total usable concentration of these additives is limited by surface mixing viscosity.
U.S. Pat. No. 4,778,528 attempts to address the problem of free water formation and suggests the addition to cement slurry compositions of 100 micromols to 5 millimols per 100 grams of cement of a soluble salt of magnesium, tin, lead, bismuth or a transition metal, or a combination of these.
U.S. Pat. No. 4,888,120 relates to the use of synthetic swellable layer silicates in water-based well drilling fluids as a high temperature stable thixotrope which are said to be substantially unaffected by alkaline earth metal ions. The patent does not deal with cement compositions which contain synthetic swellable layer silicates.
Published UK patent application GB 2,296,713 and its counterpart WO 96/20899 describe synthetic hectorite/cement mixtures which are said to be capable of rapid and reversible gelling.